At the present time, virtually all printed copy is produced through the use of three basic types of printing plates. One type is a relief plate which prints from a raised surface. Another type is an intaglio plate which prints from a depressed surface. The third type is a lithographic plate which prints from a substantially flat surface which is neither appreciably raised above nor appreciably depressed below the adjacent and surrounding non-printing areas. The present invention concerns the third type of printing plate.
Lithographic printing is occasioned by an ink's respective affinity and/or aversion to areas of different chemical properties. Lithographic printing plates are commonly processed to have water-repellent (hydrophobic), oil-receptive (oleophilic) image areas and water-receptive (hydrophilic) non-image areas.
In preparing a conventional lithographic plate for use on a printing press, the plate is first imagewise exposed to actinic radiation to effect specific chemical reactions therein. Such photoinduced chemical reactions may either reduce or enhance the solubility of the photoresist, depending on whether the resist is negative-working or positive-working. In negative-working plates, exposure to actinic radiation will generally cause a "hardening" or insolubilization of the photoresist. In positive-working plates, exposure to actinic radiation will generally cause a "softening" or solubilization of the photoresist.
After photoexposure, a wet development step is normally conducted to remove those areas of the photoresist which have undergone photoinduced chemical change or those which have not been photoexposed. Solvation under conventional development techniques will typically involve treating the exposed plate with organic solvents in a developing bath. For negative-working resists, the solvent will swell and dissolve the unexposed portions of the resist. For positive-working resists, the response of the unexposed and exposed coatings are reversed, but the same general principles apply. In either case, the solvent should not react with the unintended portions, otherwise distortion of the developed image may result.
As a result of the preferential solvation and washing away of portions of the photoresist, corresponding portions of the underlying hydrophilic substrate are uncovered. For negative-working plates, the aforementioned hydrophobic image areas correspond to the portions of the photoresist remaining after salvation and washing, whereas the aforementioned hydrophilic non-image areas correspond to portions of the substrate revealed by said solvation and washing. The image and non-image areas thus differentiated, the processed plate may then be mounted onto a printing press and run.
Encumbered by required wet development, the processing of conventional lithographic plates prior to their use on a printing press is both time and labor consuming and involves use of volatile organic chemicals. Considerable desire is thus present for means that would satisfactorily eliminate or reduce the long-felt dependency of conventional lithographic processes upon the conduct of wet development and thereby permit use of lithographic plates on a printing press immediately after exposure without required post-exposure prepress processing.
So-called "expose-only", "no-process", and "on-press developable" lithographic plates have been suggested. Among printing plates that may be characterized as such are: e.g., U.S. Pat. No. 3,793,033, issued to Mukhedjee on Feb. 19, 1974; U.S. Pat. No. 4,273,851, issued to Muzyczko et al. on Jun. 16, 1981; U.S. Pat. No. 4,408,532, issued to J. H. Incremona on Oct. 11, 1983; U.S. Pat. No. 5,258,263, issued to Z. K. Cheema, A. C. Giudice, E. L. Langlais, and C. F. St. Jacques on Nov. 2, 1993; U.S. Pat. No. 5,368,973, issued to Hasegawa on Nov. 29, 1994; U.S. Pat. No. 5,395,734, issued to Vogel et al. on Mar. 7, 1995; U.S. Pat. No. 5,506,090, issued to Gardner et al. on Apr. 9, 1996; U.S. Pat. No. 5,514,522, issued to W. C. Schwarzel, F. R. Kearney, M. J. Fitzgerald, and R. C. Liang on May 7, 1996; and U.S. Pat. No. 5,516,620, issued to L. C. Wan, A. C Giudice, J. M. Hardin, C. M. Cheng, and R. C. Liang on May 14, 1996. Mention may also be made of PCT/US95/11104; U.S. Pat. No. 5,488,147; GB 9322705; EP 599463; EP 562952; U.S. Pat. No. 5,314,785; U.S. Pat. No. 5,225,316; U.S. Pat. No. 5,235,015; U.S. Pat. No. 5,102,771; and U.S. Pat. No. 5,085,972.
While good results are obtained from current on-press developable plates, a desire has developed to extend the performance of such and like media across a wider range of environmental conditions without departing from good imaging performance. Of particular interest is the realization of such performance under varying environmental conditions, particularly tropical (i.e., hot and humid) conditions, these conditions being foreseeably encountered when such media are transported, stored, and/or used during imaging and development. Advantage may be derived from reduced implementation of environmentally-insulating packaging, transportation, and storage, as well as by ameliorating the affects on imaging of environmental conditions.